Mesh: Move material indices to a generic attribute

This patch moves material indices from the mesh `MPoly` struct to a
generic integer attribute. The builtin material index was already
exposed in geometry nodes, but this makes it a "proper" attribute
accessible with Python and visible in the "Attributes" panel.

The goals of the refactor are code simplification and memory and
performance improvements, mainly because the attribute doesn't have
to be stored and processed if there are no materials. However, until
4.0, material indices will still be read and written in the old
format, meaning there may be a temporary increase in memory usage.

Further notes:
* Completely removing the `MPoly.mat_nr` after 4.0 may require
  changes to DNA or introducing a new `MPoly` type.
* Geometry nodes regression tests didn't look at material indices,
  so the change reveals a bug in the realize instances node that I fixed.
* Access to material indices from the RNA `MeshPolygon` type is slower
  with this patch. The `material_index` attribute can be used instead.
* Cycles is changed to read from the attribute instead.
* BMesh isn't changed in this patch. Theoretically it could be though,
  to save 2 bytes per face when less than two materials are used.
* Eventually we could use a 16 bit integer attribute type instead.

Ref T95967

Differential Revision: https://developer.blender.org/D15675

3 files changed, 22 insertions, 8 deletions

diff --git a/source/blender/io/collada/GeometryExporter.cpp b/source/blender/io/collada/GeometryExporter.cpp
index 7e2a24aeb41..1a3a68923e3 100644
--- a/source/blender/io/collada/GeometryExporter.cpp
+++ b/source/blender/io/collada/GeometryExporter.cpp
@@ -17,6 +17,7 @@
 
 #include "BLI_utildefines.h"
 
+#include "BKE_attribute.hh"
 #include "BKE_customdata.h"
 #include "BKE_global.h"
 #include "BKE_lib_id.h"
@@ -284,15 +285,18 @@ static bool collect_vertex_counts_per_poly(Mesh *me,
                                            int material_index,
                                            std::vector<unsigned long> &vcount_list)
 {
+  const blender::bke::AttributeAccessor attributes = blender::bke::mesh_attributes(*me);
+  const blender::VArray<int> material_indices = attributes.lookup_or_default<int>(
+      "material_index", ATTR_DOMAIN_FACE, 0);
   MPoly *mpolys = me->mpoly;
   int totpolys = me->totpoly;
   bool is_triangulated = true;
 
   int i;
-  /* Expecting that p->mat_nr is always 0 if the mesh has no materials assigned */
+  /* Expecting that the material index is always 0 if the mesh has no materials assigned */
   for (i = 0; i < totpolys; i++) {
-    MPoly *p = &mpolys[i];
-    if (p->mat_nr == material_index) {
+    if (material_indices[i] == material_index) {
+      MPoly *p = &mpolys[i];
       int vertex_count = p->totloop;
       vcount_list.push_back(vertex_count);
       if (vertex_count != 3) {
@@ -397,13 +401,17 @@ void GeometryExporter::create_mesh_primitive_list(short material_index,
   /* performs the actual writing */
   prepareToAppendValues(is_triangulated, *primitive_list, vcount_list);
 
+  const blender::bke::AttributeAccessor attributes = blender::bke::mesh_attributes(*me);
+  const blender::VArray<int> material_indices = attributes.lookup_or_default<int>(
+      "material_index", ATTR_DOMAIN_FACE, 0);
+
   /* <p> */
   int texindex = 0;
   for (int i = 0; i < totpolys; i++) {
     MPoly *p = &mpolys[i];
     int loop_count = p->totloop;
 
-    if (p->mat_nr == material_index) {
+    if (material_indices[i] == material_index) {
       MLoop *l = &mloops[p->loopstart];
       BCPolygonNormalsIndices normal_indices = norind[i];
 
diff --git a/source/blender/io/collada/MeshImporter.cpp b/source/blender/io/collada/MeshImporter.cpp
index 34792fd6bb4..fab53908d5a 100644
--- a/source/blender/io/collada/MeshImporter.cpp
+++ b/source/blender/io/collada/MeshImporter.cpp
@@ -615,6 +615,9 @@ void MeshImporter::read_polys(COLLADAFW::Mesh *collada_mesh, Mesh *me)
 
   MaterialIdPrimitiveArrayMap mat_prim_map;
 
+  int *material_indices = (int *)CustomData_add_layer_named(
+      &me->pdata, CD_PROP_INT32, CD_SET_DEFAULT, nullptr, me->totpoly, "material_index");
+
   COLLADAFW::MeshPrimitiveArray &prim_arr = collada_mesh->getMeshPrimitives();
   COLLADAFW::MeshVertexData &nor = collada_mesh->getNormals();
 
@@ -633,7 +636,7 @@ void MeshImporter::read_polys(COLLADAFW::Mesh *collada_mesh, Mesh *me)
     int collada_meshtype = mp->getPrimitiveType();
 
     /* since we cannot set mpoly->mat_nr here, we store a portion of me->mpoly in Primitive */
-    Primitive prim = {mpoly, 0};
+    Primitive prim = {mpoly, material_indices, 0};
 
     /* If MeshPrimitive is TRIANGLE_FANS we split it into triangles
      * The first triangle-fan vertex will be the first vertex in every triangle
@@ -663,6 +666,9 @@ void MeshImporter::read_polys(COLLADAFW::Mesh *collada_mesh, Mesh *me)
           }
 
           mpoly++;
+          if (material_indices) {
+            material_indices++;
+          }
           mloop += 3;
           loop_index += 3;
           prim.totpoly++;
@@ -1007,10 +1013,9 @@ void MeshImporter::assign_material_to_geom(
 
     for (it = prims.begin(); it != prims.end(); it++) {
       Primitive &prim = *it;
-      MPoly *mpoly = prim.mpoly;
 
-      for (int i = 0; i < prim.totpoly; i++, mpoly++) {
-        mpoly->mat_nr = mat_index;
+      for (int i = 0; i < prim.totpoly; i++) {
+        prim.material_indices[i] = mat_index;
       }
     }
   }
diff --git a/source/blender/io/collada/MeshImporter.h b/source/blender/io/collada/MeshImporter.h
index 92b387a4bfe..1def84e8f99 100644
--- a/source/blender/io/collada/MeshImporter.h
+++ b/source/blender/io/collada/MeshImporter.h
@@ -80,6 +80,7 @@ class MeshImporter : public MeshImporterBase {
    * (<triangles>, <polylist>, etc.) */
   struct Primitive {
     MPoly *mpoly;
+    int *material_indices;
     unsigned int totpoly;
   };
   typedef std::map<COLLADAFW::MaterialId, std::vector<Primitive>> MaterialIdPrimitiveArrayMap;